1. Field of the Invention
The present invention relates generally to solid-state image sensing apparatus employing a CCD (Charge-Coupled Device) as a solid-state image sensing device and methods of driving the CCD and, more particularly, to a solid-state image sensing apparatus such as video camera, which electronically controls exposure by shortening and expanding a photoelectric conversion period of a CCD and to a method of driving the CCD.
2. Description of the Background Art
In image sensing apparatus such as a video camera, a CCD has conventionally been in wide use as a solid-state image sensing device. The CCDs of frame transfer type and interline transfer type are widely known. A general description will now be given on a fundamental structure of the frame transfer type CCD and on a principle of operation of the CCD.
FIG. 1 is a schematic block diagram showing a general structure of the frame transfer type CCD. A CCD 10 shown in FIG. 1 includes a light receiving portion (an image sensing portion) 11, a storage portion 12 and a horizontal transfer portion 13.
Light receiving portion 11 includes a large number of light receiving elements (photosensors) Ph arranged two-dimensionally in the directions of rows and columns. These light receiving elements are vertically connected in series for each column. Light from an object is incident on light receiving portion 11 through a lens not shown.
Storage portion 12 also includes the same number of registers arranged two-dimensionally as the light receiving elements in light receiving portion 11. These registers are vertically connected in series for each column in the manner of expanding each column of the photosensors of light receiving portion 11, to constitute a shift register. Storage portion 12 is hindered from being exposed to light unlike light receiving portion 11.
Light incident on light receiving portion 11 is photoelectrically converted into signal charges (photo charges) by light receiving elements Ph. The generated charges are stored in each light receiving element Ph for each vertical scanning period. The signal charges thus stored and corresponding to one field are transferred to storage portion 12 by multi-phase clock pulses .phi..sub.F (e.g., .phi..sub.F1 -.phi..sub.F3 in FIG. 1) which are applied to the light receiving elements from a signal source not shown, during a vertical blanking period of a video signal. Then, storage of the signal charges restarts from O in light receiving portion 11. The transfer of the signal charges from light receiving portion 11 to storage portion 12 requires two to three horizontal (H) periods in general and completes within the vertical blanking period.
The signal charges corresponding to one field thus transferred to storage portion 12 are sequentially read in horizontal transfer portion 13 in the next vertical video period. More specifically, the signal charges stored in storage portion 12 and corresponding to one horizontal scanning line are transferred to horizontal transfer portion 13 for each one horizontal period by multi-phase clock pulses .phi..sub.S (e.g., .phi..sub.s1 -.phi..sub.s3 of FIG. 1) which are applied to the registers from a signal source not shown for each horizontal blanking period.
The signal charges corresponding to one horizontal scanning line and transferred to horizontal transfer portion 13 are horizontally transferred for each horizontal period by multi-phase clock pulses .phi..sub.H (e.g., .phi..sub.H1 -.phi..sub.H3 of FIG. 1) which are applied to horizontal transfer portion 13 from a signal source not shown, and then supplied in sequence as an image signal X (t).
In solid-state image sensing apparatus employing the above-described CCD, exposure control apparatus for automatically controlling exposure by utilizing the above mentioned CCD operation principle is proposed and disclosed in, for example, Japanese Patent Laying-Open No. 63-24764. The principle of such automatic exposure control will now be described taking the frame transfer type CCD shown in FIG. 1 as an example.
That is, in the frame transfer type CCD of FIG. 1, the signal charges stored in light receiving portion 11 are transferred in the opposite direction to the direction of transfer for reading (the direction shown by the arrow in FIG. 1) at certain timing during a period of photoelectric conversion by light receiving portion 11 for each vertical scanning period, and then discharged to an drain or the like not shown. The discharge of the charges is carried out by controlling the phases of multi-phase clock pulses .phi..sub.F to be applied to light receiving elements Ph of light receiving portion 11, so that the discharge of all the charges can be completed within 2-3H periods likewise the case of reading. Then, only the charges, which are newly generated and stored in light receiving portion 11 in the remaining photoelectric conversion period (hereinafter referred to as exposure period) from the timing of the discharge to the next vertical blanking period, are transferred to storage portion 12 in the vertical blanking period and then outputted through horizontal transfer portion 13 in the above-described manner. The amount of photo signal charges to be obtained (image signal level) can be appropriately controlled by making the timing to discharge the charges of light receiving portion 11 in the opposite direction variable depending on luminance of the object, i.e., by making the remaining photoelectric conversion period (exposure period) expansible depending on the luminance of the object. That is, the control over the timing to discharge the charges makes it possible to obtain a constantly optimal exposure state.
Since the discharge of the charges in the opposite direction is carried out once for each vertical scanning period as described above, a processing for altering the timing to discharge the charges is also carried out for each vertical scanning period. In the foregoing conventional exposure control apparatus, the width of change of timing by a single processing for timing alteration is fixed at a definite value (e.g., the time corresponding to 8H periods).
When the width of a single shortening and expansion of the exposure period is fixed in a constant time, however, the ratio of the varying width of exposure period to the entire exposure period becomes different between a longer exposure period and a shorter exposure period, resulting in the following problem.
More specifically, when the width of a single shortening and expansion of the exposure period is fixed in e.g. 8H periods, the variation ratio of the exposure period in a certain vertical scanning period is 10% when the exposure period is e.g. 80H, while the ratio is 50% when the exposure period is e.g. 16H. Accordingly, the ratio of change in amount of signal charges to be obtained from the CCD as a solid-state image sensing device, i.e., in level of image signal X (t) becomes uneven for each vertical scanning period.
It has been generally confirmed by experiments that when the variation ratio of the exposure period is over 20%, a change in luminance on a reproduced picture due to this variation can be recognized by human eyes. Therefore, such uneven change of exposure period, i.e., an uneven change in level of an image signal for each vertical scanning period causes an uneven change in luminance on the reproduce picture, resulting in the reproduced picture which is extremely hard to see.